1. Field of the Invention
The field of the invention is that of carriers using information provided both by an inertial unit and by a satellite navigation system, as for example a GPS system.
The invention relates to a data fusion device and method and applies more particularly to the detection and rejection of satellite failures.
2. Description of Related Art
Carriers such as aircraft or even boats have many navigation systems available. Among these systems is included hybrid INS/GNSS (Inertial Navigation System/Global Navigation Satellite System) equipment.
An inertial unit provides low-noise information that is accurate in the short term. However, in the long term, the localization performance of an inertial unit decays (more or less quickly depending on the quality of the sensors, accelerometer or gyroscopes for example, and of the processing employed by the unit). While for its part the information acquired from a satellite navigation system is very unlikely to drift in the long term, it is however often noisy and of inconsistent accuracy. Furthermore, inertial measurements are always available while GNSS information are not or are liable to be decoyed or scrambled.
Data fusion consists of combining the information supplied by the inertial unit and the measurements provided by the satellite navigation system to obtain position and speed information by taking advantage of both systems. Thus, the accuracy of the measurements provided by the GNSS receiver allows the inertial drift to be controlled and the low-noise inertial measurements make it possible to filter the noise in the GNSS receiver measurements.
Modern navigation systems calculate protective radii about the calculated position which allow the true position error to be held to a given integrity risk, which is what defines the integrity of a system.
According to the state of the art, these protective radii can be calculated by means of a bank of Kalman filters which makes it possible to have protection from a possible satellite failure.
These filters implement the fusion of the information coming from the satellite navigation system and that coming from the inertial unit. One of the filters in the bank, designated with the term principal filter, uses all the GNSS measurements consisting of pseudo-measurements and information on their quality. The other filters, called secondary, of the filter bank use only a part of the available GNSS measurements. If a breakdown occurs in a satellite measurement, this will not be seen by the secondary filter which does not receive this measurement: this secondary filter will therefore remain uncontaminated.
The use of such a bank of Kalman filters within the scope of a closed loop data fusion (that is when it is desired to apply the corrections calculated by the Kalman filter directly to the virtual platform) has for example been proposed by document EP1801539 A1.
This document contemplates advantageously integrating only one virtual platform and to implement satellite failure detection in order to select the Kalman filter the output whereof will be applied to the virtual platform and to the inertial measurements issuing from it in order to correct them.
Thus, this document contemplates selecting the main Kalman filter when no failure of one of the satellites is detected or, when a failure is detected, to select the secondary Kalman filter not affected by the failure.
Inasmuch as the corrections thus come from a single filter and as this filter is not affected by a satellite failure, erroneous corrections are not applied to the inertial measurements through propagation of information corrupted by a satellite failure.
However, the architecture proposed in this document has not proven completely satisfactory. Indeed, inasmuch as corrections are derived from a single filter, if a failure is not detected or if the wrong satellite is rejected, the virtual platform will be compensated with a command issued by a filter contaminated by that failure. Inasmuch as this document also provides for the calculation of a priori estimated pseudo-measurements being carried out based on information delivered by the virtual platform, the pseudo-measurements used by all the filters are then contaminated.
Thus, when a failure appears in one pseudo measurement and as long as the failure detection and rejection method has not identified the faulty satellite, the filter which does not use the pseudo-measurement where the failure is actually present is contaminated.
Moreover, this architecture proposes to carry out a copying operation onto all the other filters of the system, rejecting the satellite identified as having failed by the detection and rejection method. This copying operation is problematical in the case where the satellite identified as having failed is in reality healthy because the filter which is genuinely healthy is replaced with a contaminated filter. This method can lead to a loss of integrity inasmuch as the position error committed at the output of the hybrid system may no longer be protected by the protective radius calculated by the system.
The use of a bank of Kalman filters was also proposed in document U.S. Pat. No. 7,219,013. According to this document, the protective radii are calculated using the separation solution method while the detection and rejection of a satellite breakdown implement monitoring of the innovation residuals of the Kalman filters.
But in this architecture, too, and for reasons similar to those stated earlier in connection with document EP1801539 A1, the integrity of the filter which rejects the failure is not guaranteed.
Furthermore, this architecture does not allow isolation of the breakdowns induced by slight drifts, so that it proves necessary to complete the monitoring of the innovation residuals with a so-called least squares method using only GNSS information.